Field of the Invention
The invention relates to a magnetoresistive random-access memory (MRAM), whose storage effect resides in the magnetically variable electrical resistance of the memory cell.
Magnetoresistive memories have a magnetoresistive layer system between, for example, a word line and a bit line. The layer system comprises, for example, a soft-magnetic layer and a hard-magnetic layer, which are isolated by a thin tunnel oxide. The resistance between the bit line and the word line depends, then, on whether the magnetization directions in the materials are parallel or antiparallel. Parallel magnetization leads to a lower resistance and antiparallel magnetization direction leads to a higher resistance. In many respects, the relatively high currents or current peaks, required in particular for cell writing, in the word and bit lines are disadvantageous because the current densities resulting therefrom lead to electromigration problems, and to a relatively high power loss. Furthermore, the relatively high currents mean that increased requirements are placed on the peripheral circuits. Since the materials for the bit and word lines must, for example, be process-compatible, readily patternable and have a low resistivity, electromigration problems can be avoided only to a very limited extent by a suitable choice of the line materials. The reduction of the required currents by using thinner magnetic layers encounters technological limits and causes greater reliability problems as the layer thickness decreases. Moreover, from today""s standpoint, it cannot be assumed that material-specific optimizations will make a significant contribution to the reduction of the required currents in the foreseeable future.
U.S. Pat. No. 4,455,626 discloses an MRAM whose magnetoresistive layer is situated in a gap in a thicker field concentrator layer. In that case, a memory layer and the field concentrator layer constitute a magnetic path to the magnetoresistive layer.
Furthermore, U.S. Pat. No. 6,028,786 (European published patent application EP 0 875 901 A3) discloses a magnetoresistive memory in which a material having a high relative permeability is used in order to effect a reduction in the current density required for writing.
It is accordingly an object of the invention to provide a magnetoresistive random-access memory, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which, with the smallest possible chip area, the current density in the bit and word lines is as low as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a magnetoresistive memory, comprising:
two layers formed of a material of a high relative permeability;
magnetic memory cells with associated bit lines and word lines formed between the two layers;
a filling of electrically insulating material having a large relative permeability disposed in at least one region selected from the group consisting of a region laterally between the bit lines, a region laterally between the memory cells, and a region laterally between the word lines.
In accordance with an added feature of the invention, the two layers are electrical insulators, and one of the two layers makes contact with the word line and the other of the two layers makes contact with the bit line.
In accordance with an additional feature of the invention, at least one of the two layers and/or the filling in at least one of the regions is formed of a ferrite.
In accordance with another feature of the invention, the regions are all filled with the same material.
With the above and other objects in view there is also provided, in accordance with an alternative feature of the invention, a magnetoresistive memory which comprises:
bit lines and word lines;
magnetic memory cells disposed at cross-over points of the bit lines and the word lines;
two layers of a material having high relative permeability enclosing therebetween the memory cells with the bit lines and word lines; and
a layer of an electrically insulating material having a low relative permeability between the memory cells with the bit lines and word lines and at least one of the two layers.
In accordance with a concomitant feature of the invention, the layer of the electrically insulating material having a low relative permeability fills a space between the two layers and the memory cells together with the bit and word lines in a region of the memory cells.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a magnetoresistive memory with a low current density, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.